Protonation-deprotonation switching

Figure 14. Protonation-deprotonation switching mechanism of quadratic NLO response.

Numerous processes may be linked to proton transfer and protonation/deprotonation reactions (for general descriptions of proton transfer see, for instance, [8.214-8.217]). Proton-triggered yes/no or +/- switching is contained in the ability of polyamine receptors and carriers to bind and transport cations when unproton-ated and anions when protonated also, zwitterions such as amino acids may change from bound to unbound or vice versa, when they undergo charge inversion as a function of pH. 

When covalently attached to electron transfer active subunits, the DHA-VHF couple can facilitate chemical and physical switching of electronic properties, as a result of photochemically induced rearrangement accompanied by a change in the redox potential. An interesting example of such a switching system is the compound containing a dihydroazulene component and a covalently attached anthraquinone moiety.1311 This system is able to act as a multimode switch, assisted by various processes such as photochromism, reversible electron transfer, and protonation-deprotonation reactions (Scheme 8). 

The complexation/decomplexation of a linear guest by a macrocyclic host can be exploited [ref. 7] (Fig. 10.2) to switch reversibly between two states. When the two components are dissociated, the system is in its State 0. When they are associated in the form of a [2]pseudorotaxane, the system is in its State 1. Switching between these two states can be achieved [ref. 7] by protonation/ deprotonation, by oxidation/reduction, or by photoinduced isomerization of the guest. Alternatively, the addition/removal of a sequestering agent able to... 

Morphological switching between nanotube, hexameric receptor and monomers is readily achieved by simple protonation-deprotonation reactions. This system can be described as a library of dynamic, size selective fullerene receptors whose structure... 

Scheme 2 Switching of second-order NLO response in the acetylide/vinylidene pairs upon protonation/deprotonation sequenees...

Switching the cubic nonlinearity of ruthenium alkynyl complexes by a protmi-ation/deprotonation sequence (via a vinylidene complex) was demraistrated by fs Z-scan studies at 800 nm several years ago [41]. Recently, protic and electrochemical switching were demonstrated in the ruthenium alkynyl cruciform complex 11 for which distinct linear optical and NLO behavior were noted for the vinylidene complex and the Ru(II) and Ru(III) alkynyl complexes [42]. Because the oxidation/ reduction and protonation/deprotonation procedures are independent, this system corresponds to switching by orthogonal stimuli. 

Kolomiets, E. Berl, V. Odriozola, 1. Stadler, A.-M. Kyritsakas, N. Lehn, J.-M. Contrac-tion/extension molecular motion by protonation/deprotonation induced structural switching of pyridine derived oligoamides. Chem. Commun. 2003, 2868-2869. 

In conclusion it is worthwhile to note that APA is a unique example of azide whose photochemieal activity can be switched by protonation/deprotonation. This enables an observed quantum yield of the azide photodissociation to be smoothly changed within two orders of magnitude. 

Chemical off—on switching of the chemiluminescence of a 1,2-dioxetane (9-benzyhdene-10-methylacridan-l,2-dioxetane [66762-83-2] (9)) was first described in 1980 (33). No chemiluminescence was observed when excess acetic acid was added to (9) but chemiluminescence was recovered when triethylamine was added. The off—on switching was attributed to reversible protonation of the nitrogen lone pair and modulation of chemically induced electron-exchange luminescence (CIEEL). Base-induced decomposition of a 1,2-dioxetane of 2-phen5l-3-(4 -hydroxyphenyl)-l,4-dioxetane (10) by deprotonation of the phenoHc hydroxy group has also been described (34). 

The dendrimer-type tetranuclear Ru(II)-Os(II)3 complex (22, protonated form) shows an interesting electrochemical behavior due to the presence of free basic sites in its bridging ligands [41]. The protonated form shows a 3-1 oxidation pattern due to the simultaneous oxidation of the three Os-based units, followed by the one-electron oxidation of the Ru-based unit. On addition of base, the six chelating moieties (three on the Ru center and one on each Os center) undergo deprotonation. This causes changes in the oxidation potential of the metal ions, with a consequent switching from 3-1 to 1-3 in the oxidation pattern. 

An interesting new bridged complex, [Ru(TDBOHPP) L a L] (type H) in which the bridge L a L is 4,4 -azopyridine, has been studied in the search for molecular switches [217]. Protonation of the polymer induces partial oxidation of the Ru(II) to Ru(III) at the expense of the azo groups which are reduced to hydrazo species. Along with the formation of the Ru(II)-Ru(III) mixed valence compound a NIR intervalence band is switched on . The chemistry of these complexes is further complicated by the phenolic hydroxy groups in the porphyrin ligand which can also be deprotonated and oxidized. 

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